Solder pastes are bonding materials formed by mixing a solder powder with a pasty flux. Solder pastes are applied to a workpiece such as a printed circuit board or a silicon wafer by screen printing using a mask or dispensing through a syringe so as to deposit a suitable small amount of the paste on each area to be soldered of the workpiece and are subsequently heated in a furnace to melt the solder and adhere it to the workpiece. This soldering technique is generally called reflow soldering. Various heating methods including infrared heating, laser heating, hot air heating, and hot plate heating can be employed in the furnace (which is also called a reflow furnace). Solder pastes are required to have rheological properties suitable for screen printing or dispensing.
Generally, a flux for a solder paste is prepared by dissolving solid constituents such as rosin, a thixotropic agent, and an activator in a liquid solvent so as to provide the resulting flux with suitable rheological properties. When reflow soldering is carried out with a conventional solder paste having a flux which comprises solid constituents dissolved in a solvent, a considerable part of the solid constituents of the flux remains on or in the vicinity of the resulting soldered joints (i.e., in the soldered areas) as a flux residue.
The flux residue, if remaining in a significant amount in the soldered areas, worsens the appearance of the soldered areas and impedes the contact of the pins of an electronic part to be soldered with the solder paste. Furthermore, the flux residue may cause the insulation resistance between circuits of the workpiece to decrease due to moisture absorption by the flux residue or may cause the circuits to be broken or disconnected by the formation of corrosion products from the flux residue. Therefore, workpieces intended for use in electronic equipment for which high reliability is required have to be washed with a cleaner, after reflow soldering, in order to remove the flux residue remaining in the soldered areas to attain good adhesion of the resin.
In the case of electronic equipment for use in motor vehicles, resin coating in which the soldered areas are coated with a resin or resin molding in which the entire workpiece is sealed in a resin molding is frequently applied using a moisture-proofing resin in order to guarantee the reliability of the equipment. Also in such a case, the workpiece must be washed with a cleaner to remove any flux residue prior to resin coating or molding.
Fluorinated solvents and chlorinated solvents have been used as a cleaner to remove flux residues since they can readily dissolve rosin in the flux residues. However, the use of these solvents is now regulated since their vapors cause ozone depletion, thereby increasing the amount of ultraviolet rays which reach the earth from the sun. This increase is undesirable because ultraviolet rays are harmful and may induce skin cancer in humans.
Furthermore, with an increase in the package density, the spacing between adjacent electronic parts mounted on a workpiece becomes narrow, thereby making it difficult to completely remove flux residues by washing.
A decreased residue solder paste which needs no cleaning after reflow soldering and which is suitable for use in soldering of workpieces to be incorporated into electronic equipment for which high reliability is required is described in U.S. Pat. No. 5,176,759. That decreased residue solder paste, which contains a decreased amount of rosin, does not completely eliminate the formation of a flux residue, but it leaves a flux residue in an amount that is harmless in normal applications.
The decreased flux residue remaining after reflow soldering with the decreased residue solder paste causes no substantial problems in ordinary electronic equipment for which high reliability is required. However, the decreased residue solder paste is not completely satisfactory for present-day electronic equipment for which ultrahigh reliability is required.
Thus, there is a need for a residue-free solder paste, which makes it possible to leave at most 1% of the flux as a flux residue after reflow soldering and which does not cause any adverse effects of a flux residue without being washed with a cleaner.
Rosin is a natural resin from pine trees which comprises abietic acid and its isomers as main constituents. A conventional solder paste for use in electronic equipment is prepared from a rosin-based flux, and rosin has been considered to be an essential flux constituent in view of its effectiveness for modifying the viscosity of the paste and improving the retention of electronic parts by tacky adhesion, as well as in view of the reducing activity of abietic acid and its isomers, the main constituents of rosin, to remove oxides. However, the presence of rosin in a flux for solder paste inevitably results in the formation of a flux residue after reflow soldering in which most constituents of the rosin remain in the residue. Accordingly, rosin cannot be used as a flux constituent in a residue-free solder paste for which it is desired that 99% or more of the flux not remain after reflow soldering.
Some residue-free solder pastes which do not contain rosin have been proposed in the prior art.
U.S. Pat. No. 4,919,729 discloses a fluxless solder paste for use in a reducing atmosphere formed by mixing a solder powder and a binder, which is a monohydric or polyhydric compound having a boiling point between the solidus and liquidus temperatures of the solder powder.
JP H02-290693 A1 discloses a solder paste comprising solder microparticles and an alcohol having a boiling point higher than the melting point of the solder. The solder paste may contain a vaporizable activator, a methyl methacrylate oligomer, and a fatty acid amide. Only liquid solvents are used in the solder pastes disclosed therein.
U.S. Pat. No. 5,919,317 describes a soldering flux and a solder paste which leaves substantially no residue after reflow soldering in a non-reducing atmosphere. The soldering flux contains a polyhydric organic compound having a temperature at which the mass % becomes approximately 0%, as measured by thermal gravimetry, of at least 170° C. and above the solidus temperature of a solder used in the solder paste.
In order to make a solder paste residue-free, the solder paste must be freed of rosin, while maintaining properties, which include printability and dispensability (transferability), retainability of parts (by tacky adhesion), wettability, and storage stability, as good as are attainable with a conventional rosin-based solder paste, in the absence of rosin. Furthermore, if the flux contains any solid or highly viscous constituents such as a thixotropic agent or an activator, such constituents must be vaporized during reflow soldering such that they do not form a flux residue.
None of the residue-free solder pastes proposed in the above-described documents are sufficient in rheological properties. Thus, their viscosities and thixotropic properties are not sufficiently high. As a result, these solder pastes may have the problem that the solder powder in the solder paste tends to separate from the flux, thereby adversely affecting the storage stability and printability of the solder paste through a metal mask and making it difficult to dispense the paste through a syringe. In addition, they do not have sufficient retainability of parts.